Field of the Invention
The present invention relates to the field of turbine engines and more particularly to a bearing for guiding a shaft of a turbine engine.
Description of the Related Art
The guide bearings used in a turbine engine comprise an inner ring and an outer ring enclosing rolling members, for example rollers. Conventionally, the outer ring is mounted in a connected manner on a stationary portion of the turbine engine and the inner ring is mounted in a connected manner on a shaft of the turbine engine, for example by means of press fitting. The bearing thus makes it possible to rotatably guide the shaft in relation to the stationary portion of the turbine engine.
In certain configurations, the bearing is mounted between two rotary shafts of the turbine engine. This type of bearing is commonly referred to as an “intershaft” bearing, a bearing of this type being known for example from the patent application FR 2 939 843 A1 by the company SNECMA.
By way of example, with reference to FIG. 1, a turbojet engine for an aircraft conventionally comprises a plurality of rotary turbine shafts, including a high-pressure shaft 2 which is rotatably mounted in relation to a low-pressure shaft 1 via an intershaft bearing 3 comprising rollers 33. The shafts are coaxial and extend along the axis X of the turbojet engine, the low-pressure shaft 1 being mounted inside the high-pressure shaft 2. The low-pressure shaft 1 comprises a downstream end which has a guide chute enveloping the downstream end of the high-pressure shaft 2 such that the bearing 3 is located in the chute between the outer surface of the high-pressure shaft 2 and the inner surface of the low-pressure shaft 1. In other words, the outer ring 32 of the intershaft bearing 3 is integral with a portion of the low-pressure shaft 1, whilst the inner ring 31 of the intershaft bearing 3 is integral with a portion of the high-pressure shaft 2, the rollers 33 of the bearing 3 being enclosed by the rings 31, 32 In this example, the inner ring 31 is press-fitted onto the high-pressure shaft 2 so as to prevent any translation and any rotation of the inner ring 31 of the bearing 3 in relation to the high-pressure shaft 2.
The turbojet engine moreover comprises means for supplying a flow of lubrication oil F for lubricating the guide bearing 3, which means are located inside the low-pressure shaft 1. In this example, the supply means are in the form of jets, but they may of course be in various forms. In order to lubricate the rollers 33 of the intershaft bearing 3, the inner ring 31 comprises radial channels 34 which make it possible to guide the lubrication oil F from the inner surface of the inner ring 31 to the outer surface thereof. Under the effect of the centrifugal forces, the flow of lubrication oil F successively passes through the wall of the low-pressure shaft 1 via a radial through-hole 11, the wall of the high-pressure shaft 2 via a radial through-hole 29 and lastly the inner ring 31 of the bearing 3 via radial through-channels 34 in order to reach the rollers 33 of the bearing 3.
With reference to FIG. 1, the high-pressure shaft 2 comprises, upstream thereof, solid turbine discs 28 which are sensitive, on the one hand, to the centrifugal forces as a result of the mass thereof, and, on the other hand, to the thermal expansion, as a result of the proximity thereof to the combustion chamber of the turbojet engine. Since the upstream portion of the high-pressure shaft 2 is in contact with the hot gases emanating from the combustion chamber and the downstream portion is cooled by a flow of lubrication oil, a thermal gradient, of approximately 200° C., appears between the upstream portion and the downstream portion of the high-pressure shaft 2.
During the operation of the turbojet engine, the high-pressure shaft 2 deforms under the combined effects of the centrifugal forces and the thermal expansion. Conventionally, it is said that the high-pressure shaft 2 “becomes conical”, given that the diameter of the upstream portion thereof increases whilst the downstream diameter thereof remains constant as shown in FIG. 2. In other words, the upstream portion of the high-pressure shaft 2 pivots radially by an angle α as shown in FIG. 2.
The conical shaping of the high-pressure shaft 2 leads to an orientation defect of the inner ring 31 of the bearing 3 which is mounted in a connected manner on the high-pressure shaft 2 as shown in FIG. 2. Since the inner 31 and outer 32 rings of the bearing 3 are no longer parallel, this results in mechanical stress at the rollers 33 of the bearing 3 and lubrication and cooling defects.
An immediate solution to eliminate this disadvantage would be to extend the upstream portion of the high-pressure shaft 2 in order to limit the pivoting thereof by the angle α. This solution is however to be avoided, since it contradicts the evolutions of the turbojet engines which aim to reduce the mass of the turbojet engines as well as the size thereof.